1. Field of the Invention:
This invention relates generally to an amplifier circuit having two negative feedback circuits of different negative feedback ratios, for example, which can be employed as a pre-amplifier for an audio or video magnetic tape recorder requiring different gains for recording and reproduction, and more particularly to such an amplifier circuit which can be constructed as a semiconductor integrated circuit.
2. Description of the Prior Art:
FIG. 1 shows a prior art amplifier circuit having two negative feedback circuits of different negative feedback ratios, for example, which is employed as a pre-amplifier for an audio magnetic tape recorder requiring two gains for recording and reproduction. In the amplifier circuit shown in FIG. 1, a semiconductor differential amplifier is formed from first and second NPN transistors 10, 20 having a base electrode 11, 21, an emitter electrode 12, 22, and a collector electrode 13, 23, respectively. The emitter electrodes 12, 22 of the two transistors 10, 20 are connected to each other. The common emitter electrodes 12, 22, are grounded through a common emitter current source 30. The collector electrode 13 of the first transistor 10 is connected to a positive voltage source +V.sub.1 (not shown in FIG. 1). The collector electrode 23 of the second transistor 20 is connected to the positive voltage source +V.sub.1 through a load resistor 40. The base electrode 11 of the first transistor 10 is connected to an input terminal 50 of the amplifier circuit. The base electrode 21 of the second transistor 20 is connected to the negative feedback circuit 80, to be discussed. The semiconductor differential amplifier described above is driven by an input signal applied to the input terminal 50 and the common emitter current source 30. The output voltage of the semiconductor differential amplifier corresponding to the input signal appears across the load resistor 40 and is applied to an amplifier 60 of the next stage. A portion of the output signal from an output terminal 70 of the amplifier 60, which is connected to an output terminal of the amplifier circuit, is fed back to the base electrode 21 of said second transistor 20 through the negative feedback circuit 80. The negative feedback circuit 80 has two feedback loops. One of the two feedback loops is the series connection circuit comprising a first resistor 81, a second resistor 82 and a first capacitor 83. The other of the two feedback loops is the series connection circuit comprising a third resistor 84, a parallel circuit of a fourth resistor 85 and a second capacitor 86, the second resistor 82 and the first capacitor 83. One terminal of the first capacitor 83 is grounded. One terminal of the first and third resistor 81, 84 are selectively connected to the output terminal 70 of the amplifier 60 during recording and reproduction of audio signals by an electrical switching means 90. The base electrode 21 of the second transistor 20 is connected to the connection point between the first resistor 81 (or the parallel circuit of the fourth resistor 85 and the second capacitor 86 and the second resistor 82 as is shown in FIG. 1.)
As the negative feedback circuit 80 comprises two feedback loops, the amplifier circuit described above can produce two different gains and two gains having different frequency characteristics. However, as the electrical switching means 90 is connected between the out terminal 70 of the amplifier 60 and the negative feedback circuit 80, the potential at the electrical switching means 90 relative to ground varies continuously due to the output signal at the output terminal 70 of the amplifier 60. Therefore, it is difficult to form the amplifier circuit, particularly the electrical switching means, from a semiconductor integrated circuit.